Phased array ultrasound imaging systems assume a constant acoustic velocity in human tissue of 1540 meters per second (m/s) which is used to compute delays for steering and focusing the ultrasound beam. However, due to tissue inhomogeneities and varying tissue thicknesses the different components of the ultrasound beam arrive at the focus out of phase. One way to correct these phase errors is to adjust the electronic phase delay of each element to compensate for the aberrating tissue. The process of restoring the ultrasound beam focus by correcting the phase errors is called phase correction.
Algorithms for real-time ultrasound phase correction are known. One method, by Flax et al., (IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 35, 758-767, 1988), uses cross-correlation to extract the phase error profile. In this method, individual phase differences between adjacent elements are calculated. The calculated phase error at each element is then a summation of the individual phase differences across the array. By subtracting the calculated phase error for each element from the electronic phase delays, the ultrasound beam is restored.
Another method originally described by Nock et al., (The Journal of the Acoustical Society of America, vol. 85, 1819-1833, 1989) and recently improved by Ng et al. (IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 41, 631-643, 1994), uses speckle brightness as a quality factor to determine the phase error profile. In this method, an iterative process maximizes the speckle brightness for a given region and the corresponding phase error profile for that region is then calculated. Once again, the ultrasound beam is restored by subtracting the calculated phase error for each element from the electronic phase delay.
Ultrasound phase aberration in tissue is present in two dimensions. Freiburger et al. (Ultrason. Imaging 14, 398-414, 1992) reported an average root mean square (rms) magnitude of 55.3 nanoseconds (ns) for two-dimensional phase aberration profiles which were measured across the breast. Sumino et al. (The Journal of the Acoustical Society of America, vol. 90, 2924-2930, 1991) measured arrival time differences through excised abdominal wall tissue and found an average standard deviation of 25.6 ns.
In order to correct the total phase aberration in tissue, a two-dimensional correction should be implemented. Since phase correction methods in ultrasound compensate for the aberrating tissue by adjusting the computed electronic phase delays of the array, a two-dimensional array would be required to completely correct the two-dimensional aberrations in tissue.
Unfortunately, a simple 3.5 MHz two-dimensional array capable of two-dimensional phase correction may consist of 128 elements in azimuth by at least 4 elements in elevation which leads to a total of 512 elements in a 28 millimeter (mm) by 8 mm area. This large number of elements requires an equivalent number of effective channels in the ultrasound system. Furthermore, the small element size means that each element has a high electrical impedance which reduces the sensitivity compared to larger array elements. Goldberg et al. (IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 41, 761-771, 1994). The large number of elements and reduced sensitivity of two-dimensional arrays make them an unattractive option for phase correction for reasons of cost, electrical power consumption, and length of signal processing time of the system.